This invention relates to an improved process for isolating technically pure 2,4-diisocyanatotoluene or isomer mixtures having an increased content of 2,4-diisocyanatotoluene from isomer mixtures of 2,4- and 2,6-diisocyanatotoluene by single-stage crystallization in a tube crystallizer.
Mixtures of 2,4- and 2,6-isomers generally accumulate during the commercial production of diisocyanatotoluene (an important starting material for the production of polyurethane plastics). However, the two isomers do not behave the same way chemically during further processing. This is evidenced by the fact that products having different properties are formed when the isomer composition of the isomer mixtures is varied. In many cases, it is advisable to use technically pure 2,4-diisocyanatotoluene (i.e. 2,4-diisocyanatotoluene having a purity of at least 98 wt. %) in the production of polyurethane plastics.
Since adjustment of the particular isomer ratio and use of the isomer mixture without an additional purification step are both complicated and expensive, there have been many attempts to achieve technically workable methods for isomer separation, particularly on the basis of crystallization.
For example, U.S. Pat. No. 3,217,024 describes a two-stage crystallization process in which a 90-94% 2,4-diisocyanatotoluene and a 65/35-isomer mixture are obtained from an 80/20-isomer mixture in the first step. The still impure 2,4-isomer (90-94%) is brought to the required purity in a second step. This complicated procedure appears to be necessary in view of the pronounced tendency of 2,4-diisocyanatotoluene to crystallize from isomer mixtures only after radical super cooling accompanied by the formation of inclusions. U.S. Pat. No. 4,246,187 discloses a process which attempted to overcome these difficulties by allowing only 10 to 20 wt. % of the 2,4-diisocyanatotoluene to crystallize out. This crystallized fraction was subsequently separated by means of a centrifuge. This process for separation or enrichment of the isomers also involves considerable technical outlay.
The so-called "drip process" described in "Chemische Technologie", 3, Carl Hanser Verlag, Munich, (1959), pages 812-813, for the separation of isomers from nucleus-chlorinated compounds cannot be satisfactorily applied to the separation of diisocyanatotoluene isomers because cooling to the softening point of the eutectic results in unsatisfactory isomer separation and poor volume/time yields.